The present relates to intrusion detectors, and particularly to infrared intrusion detectors and methods for detecting an intrusion in a protected space.
One known form of intrusion detector includes an infrared radiation sensor and a transparent window formed with a plurality of lens elements each oriented to receive infrared radiation from an infrared radiation source within a predetermined viewing zone of the protected space. The sensor includes a pair (sometimes a quad) of sensor segments mounted close to each other and electrically connected in a bucking relationship for each viewing zone defined by a lens element, such that a radiation source produces equal but opposite, and therefore zero, electrical outputs from the pair (or quad) of radiation sensing elements canceling each other if the source is non moving, but non-zero outputs if the source is moving. Accordingly, the sensor ignores non-moving infrared radiation producing objects, such as heat radiators within the protected space, sunlight entering the protected space, etc., and detects only moving infrared radiation moving objects by outputting a signal corresponding to the velocity of movement of the objects.
Such detectors, however, are prone to false alarms. The problem of false alarms is particularly present where the protected space may include a moving household pet, which should he ignored by the detector system. However, existing detector systems have difficulty in distinguishing between a small household pet to be ignored by the system, and an intruder to actuate the alarm. This is particularly true when the household pet is close to the sensor, and the intruder is distant from the sensor.
Another source of false alarms is the presence of thermal disturbances within the monitored space, such as curtains which may flutter near a sensor, heaters or other machines generating heat, etc. Existing detector systems have difficulty in distinguishing between such thermal disturbances (to be ignored by the system) and an intruder (to actuate the system) and therefore may produce false alarms.
Frequent false alarms decrease the integrity of the detector system, and may even prompt the user to ignore the alarm or disconnect the system. On the other hand, reducing the sensitivity of the detector system to decrease false alarms reduces its sensitivity to detect intruders.
According to one aspect of the present invention, there is a provided an intrusion detector for detecting an intrusion into a protected space overlying a predetermined surface, comprising: a housing to be mounted over the surface and including an infrared radiation sensor for sensing moving infrared radiation sources, and a transparent window thereover, a plurality of lens elements formed in the transparent window each oriented to receive infrared radiation from a moving infrared radiation source within a predetermined viewing zone of the protected space and to transmit infrared radiation from said source to the sensor; an alarm; and a control system for receiving the electrical output of the sensor and for outputting an alarm signal to the alarm when an infrared radiation source is detected simultaneously within at least two of the predetermined viewing zones.
According to further features in the described preferred embodiment the lens elements are oriented such that their viewing zones define a plurality of viewing planes at different inclinations with respect to the surface, with the viewing zones in the viewing planes of smallest inclinations detecting moving infrared radiation source most distant from the sensor. Each of the lens elements produces approximately the same magnitude of electrical output from the sensor irrespective of the distance of the infrared radiation source from the sensor.
More particularly, in the described preferred embodiment, the lens elements are designed such those having more distant viewing zones have larger optical gains in order to produce approximately the same magnitude of electrical output from the sensor irrespective of the distance of the moving infrared radiation source from the sensor. For this purpose, the lens elements are arranged according to a rectangular matrix of a plurality of horizontal rows, each defining one of the viewing planes, and a plurality of vertical columns, each defined by one of the lens elements in all the horizontal rows. The lens elements are of the same effective optical height in each horizontal row, but are of decreasing effective optical height from the uppermost horizontal row to the lowermost horizontal row, such that the lens elements in the uppermost horizontal row have the largest optical gain, and the lens elements in the lowermost horizontal row have the smallest optical gain.
According to another aspect of the present invention, there is provided an intrusion detector for detecting an intrusion into a protected space overlying a predetermined surface, comprising: a housing to be mounted over the surface and including an infrared radiation sensor for sensing moving infrared radiation sources, and a transparent window thereover; the transparent window being formed with a plurality of lens elements each oriented to receive infrared radiation from, a moving infrared radiation source within a predetermined viewing zone of the protected space and to transmit the radiation to the sensor to cause the sensor to produce an electrical output; an alarm; and a control system for receiving the electrical output of the sensor and for outputting an alarm signal to the alarm; the infrared radiation sensor having a low transfer function at high frequencies; the control system including an active filter circuit for amplifying the output signal from the sensor at a higher amplitication gain for high frequencies than for low frequencies.
According to a further aspect of the present invention, there is provided an intrusion detector for detecting an intrusion into a protected space overlying a predetermined surface, comprising: a housing to be mounted over the surface and including an infrared radiation sensor for sensing moving infrared radiation sources, and a transparent window thereover; a plurality of lens elements formed in the transparent window each oriented to receive infrared radiation from a moving infrared radiation source within a predetermined viewing zone of the protected space and to transmit infrared radiation from the source to the sensor; an alarm; and a control system for receiving the electrical output of the sensor and for outputting an alarm signal to the alarm when a moving infrared radiation source is detected within a viewing zone; the control system generating a dynamic alarm threshold for the alarm signal, which alarm threshold dynamically varies according to the environmental noise conditions in the monitored space.
According to a still further aspect of the present invention, there is provided an intrusion detector for detecting an intrusion into a protected space overlying a predetermined surface, comprising: a housing to be mounted over the surface and including an infrared radiation sensor for sensing moving infrared sources, and a transparent window thereover; the transparent window being formed with a plurality of lens elements each oriented to receive infrared radiation from a moving infrared radiation source within a predetermined viewing zone of the protected space and to transmit the radiation to the sensor to cause the sensor to produce an electrical output therefrom; an alarm; and a control system for receiving the electrical output of the sensor and for outputting an alarm signal to the alarm; the infrared radiation sensor and control system being mounted on a printed circuit board within the housing, which printed circuit board is adjustable with respect to the transparent window by means or a slot formed in the printed circuit board movable with respect to a pin extending through the housing and the slot; the pin being rotatable to a first position to free the printed circuit board for adjustment along the slot, or to a second position to lock the printed circuit board in its adjusted position.
According to yet another aspect of the present invention, there is provided a method of detecting an intrusion into a protected space overlying a predetermined surface, comprising: mounting over the protected space an infrared radiation sensor capable of sensing moving infrared radiation sources; providing the sensor with a plurality of discrete viewing zones within the protected space; detecting a moving source of infrared radiation within any of the viewing zones; and producing an alarm signal when a moving source of infrared radiation is detected simultaneously in at least two of the viewing zones.
As will be described more particularly below, an intrusion detector may be constructed in accordance with some or all of the foregoing features to provide relatively high reliability in detecting intrusions and a relatively low rate of false alarms especially with respect to household pets and/or thermal disturbances within the monitored space.
Further features and advantages of the invention will be apparent from the description below.